1. Field of the Invention
This invention relates to a device for controlling the volume of intake air for an engine by controlling the opening of a throttle valve and, particularly, to a device for controlling the volume of intake air for an engine by controlling the opening of a throttle valve using a motor.
2. Description of the Prior Art
In a general car engine control device, a throttle valve is provided in an inlet passage for sucking air to be supplied to an engine. The throttle valve opens or closes in response to the operation of an accelerator by a driver to control the volume of intake air for the engine in accordance with the operation amount of the accelerator. The control of the volume of intake air for the engine can be achieved by connecting the throttle valve and the accelerator by mechanical connection means such as a link or wire. However, in a connection method using such mechanical connection means, the relationship between the operation amount of the accelerator and the opening of the throttle valve is fixed, there is no freedom in the control of the throttle valve, and the positional relationship between the throttle valve and the accelerator is restricted, whereby the installation position of the connection means is limited when installed on an automobile.
A constant-speed running control device, traction control device and the like have recently been installed on an automobile to automatically control the running of the automobile. Therefore, the throttle valve must be controlled independently of the operation of the accelerator by the driver, and the control of the throttle valve is carried out by electrically connecting the throttle valve to a motor or the like. For example, Japanese Laid-open Patent Application No. Hei 1-315641 discloses a method for controlling the opening of a throttle valve using a brushless motor. When a motor having a brush commutator is used, a hysteresis torque is generated in a direction opposite to the normal direction of a rotor by the pressure of a brush rectifier, thereby making it difficult to control the position of a throttle valve. The method of the above publication is to control the opening of the throttle valve by driving the brushless motor stepwise. Japanese Laid-open Patent Application No. Hei 5-240070 discloses a technology for improving the controllability of a throttle valve by connecting the rotor of a brushless motor and the rotating shaft of a throttle valve through a speed reducer or gear. This publication also discloses a technology for switching a current to be supplied to the phase of a brushless motor with a reverse voltage detector for detecting a reverse voltage generated in the stator coil (to be referred to as "phase" hereinafter) of the brushless motor or a current change detector without using a high-precision rotation detector.
However, in the above-described prior art device for controlling the volume of intake air for an engine by controlling a throttle valve, as a reverse voltage detector or current change detector is required to change the energization phase of a brushless motor, a throttle actuator constituted by the throttle valve and the brushless motor for driving the throttle valve is complicated in structure and large in size, and the number of components of the signal input interface section of a controller for controlling the motor increases.
When the energization phase is changed using only the output of a throttle opening sensor, the displacement of the switching position of the energization phase occurs due to the tolerance of the characteristics of a speed reducer and throttle opening sensor.
Further, when the energization phase is changed from one to another based on the output of the reverse voltage detector or the current change detector to drive the brushless motor, a current running through the phase abruptly changes. Therefore, when the signal of the detector is shifted by a change in a magnetic field applied to the phase, the generated torques of the motor become discontinuous, thereby changing the opening of the throttle abruptly. To cope with this problem, a three-phase energization system for supplying energization currents to the U, V and W phases of the brushless motor by independent sinusoidal waves can be employed. However, a detector for measuring the rotation angle of the motor accurately is required for this three-phase energization system, whereby the throttle actuator becomes complicated in structure and large in size.